1. Field of the Invention
The present invention relates to a communication system for performing transmission/reception via a transmission medium, such as a human body.
2. Description of the Related Art
In conjunction with technological development in recent years, a communication method using an electric field induced at a transmission medium, such as a human body, has been proposed as an entirely new communication system. One example of such a communication system is disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 11-509380.
The scheme disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 11-509380 features the ability to perform communication via a thin insulator since a signal is transmitted through capacitive coupling. On the other hand, when the transmission medium or an electrode also has a large area, the scheme has an aspect that the signal is transmitted upon approach to a certain distance even without contact.
However, when a large number of people use the above-described single communication system in a small area, a problem of crosstalk occurs because of the feature of the signal being transmitted even without contact. For applications for communicating ID (identification) information as various types of key, the signal being undesirably transmitted only upon approach is not preferable in terms of security.
In this electric-field communication technology, when both of the transmitter and the receiver are wearable equipment, it has been known that the communication becomes less tolerable to noise since only a very weak signal can be received. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 11-509380 proposes use of spectral spreading for enhancing noise tolerance. The use of the spectral spreading improves the noise tolerance and also allows for demodulation even when the signal is slightly attenuated, but the feature of the signal being transmitted even without contact is also strengthened. Thus, the problem of error due to crosstalk and so on and the signal being undesirably transmitted are further emphasized. Accordingly, in the electric-field communication, there is no precedent for practical use of spectral spreading.